The present invention relates to a method and an apparatus for separating acidic gases from syngas.
The composition of the conventional crude gas produced by coal gasification includes hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2) as the main components, and also includes nitrogen (N2), methane (CH4), hydrogen sulfide (H2S), and the like. When CO2 is removed from such a syngas to recover CO2 therefrom, first, CO in the crude gas needs to be converted into H2 and CO2 by a reaction with steam (shift reaction). In addition, when the gas after purification is used as a raw material for chemicals such as ammonia or as a fuel for power generation, acidic gases such as H2S need to be removed from the crude gas.
When CO shift reaction is performed without the removal of H2S from crude gas, the gas after the shift reaction contains both CO2 and H2S, each of which then needs to be separated and recovered individually. Conventionally, in this separation process, a physical absorption process in which H2S is selectively dissolved in a solvent is employed first to separate CO2 and H2S from each other. FIG. 1 shows the configuration of an apparatus employing a typical physical absorption process for separating acidic gases from crude syngas.
FIG. 1 is a plan view showing the schematic configuration of H2S removal means and CO2 removal means in one embodiment of a conventional acidic gas separation apparatus using a physical absorption process. As shown in FIG. 1, shifted gas after the CO shift reaction is introduced into an acidic gas absorption tower 101, and brought into contact with a solvent fed by a pump 102d. As a result, H2S is removed. The solvent having absorbed H2S in the crude gas in the acidic gas absorption tower 101 is introduced into an acidic gas stripping tower 106 through an acidic gas concentrating tower 104. In the acidic gas stripping tower 106, the absorbed acidic gas is stripped from the solvent by heating with a reboiler 109. The stripped acidic gas is exhausted as acidic gas after passing through a condenser 107. On the other hand, the solvent from which the acidic gas has been stripped is introduced into a CO2 absorption tower 111 through a pump 102b, a heat exchanger 103, and a cooler 110a. 
The crude gas from which H2S has been removed in the acidic gas absorption tower 101 is introduced into the CO2 absorption tower 111, and again brought into contact with the solvent. As a result, CO2 is removed. The purified gas after the CO2 removal is used as a fuel for power generation, a raw material for chemical synthesis, or the like. The solvent having absorbed CO2 is separated into gas components and the solvent in flash drums 112a, 112b, and 112c. The gas components from the flash drum 112a are returned to the CO2 absorption tower 111, whereas the gas components from the flash drums 112b and 112c are exhausted as CO2. The solvent from which the gas has been released returns to the CO2 absorption tower 111 through a pump 112e and a chiller 110c, and is reused in the CO2 absorption tower 111.
As described above, the conventional acidic gas separation method using physical absorption is a method in which the crude gas after the coal gasification is washed with a solvent to thereby remove H2S, and the crude gas after the H2S removal is supplied into the CO2 absorption tower, where the solvent again removes CO2.
The physical absorption process is characterized that H2S and CO2 are separated and recovered by the same solvent; however, the physical absorption process has a problem that the solvent needs to be cooled to a low temperature by a chiller, and hence power required by the chiller is extremely large.
Furthermore, the method is advantageous in requiring no heat to strip CO2 from the solvent having absorbed CO2, and in that 90% or more of carbon out of the total carbon (CO+CO2+CH4) after coal gasification can be recovered as CO2 by lowering the pressure. On the other hand, the method has a problem that the purity of the recovered CO2 is low when compared with that obtained in the case of a chemical absorption process to be described later. This is because CO2 is dissolved in proportion to the CO2 partial pressure in the crude gas, and CO, H2 and the like dissolved along with CO2 are stripped simultaneously in the striping of CO2.
On the other hand, unlike the physical absorption process, the chemical absorption process using a solvent containing an organic amine or the like is not capable of separating and recovering H2S and CO2 individually from crude gas after the CO shift reaction. For this reason, it is necessary to first separate H2S before the CO shift reaction, that is, under a condition in which the CO2 partial pressure in the crude gas is low, then to perform the CO shift reaction, and then to remove CO2 by chemical absorption.
FIG. 2 is a plan view showing the schematic configuration of H2S removal means and CO2 removal means in one embodiment of a conventional acidic gas separation apparatus using a chemical absorption process. As shown in FIG. 2, crude gas having been subjected to dust removal with a scrubber is introduced into an acidic gas absorption tower 201, where H2S in the crude gas is separated and removed. The solvent used for the removal of the acidic gas is introduced into an acidic gas stripping tower 204, through a pump 202a and a heat exchanger 203a. The acidic gas is stripped from the solvent by heating with a reboiler 207a in an acidic gas stripping tower 204, and is exhausted through a condenser 205a. On the other hand, the solvent from which the acidic gas has been stripped is supplied into the acidic gas absorption tower 201, and is reused. The crude gas from which the acidic gas has been removed in the acidic gas absorption tower 201 is introduced into a shift reactor 7, where CO in the gas is converted into CO2 by the shift reaction. Next, the crude gas after the shift reaction is introduced into a CO2 absorption tower, where CO2 is separated and removed. Thereafter, the gas is exhausted as purified gas. From the solvent having absorbed CO2 in the CO2 absorption tower 208, CO, H2, and the like dissolved in the solvent are stripped by flash in a high pressure flash drum 209, and then the solvent is introduced into a CO2 stripping tower 210. CO, H2, and the like stripped in the high pressure flash drum 209 join, after passing through a compressor 213, the crude gas after the shift reaction, and is introduced again into the CO2 absorption tower 208. CO2 stripped from the solvent in the CO2 stripping tower is exhausted through a condenser 205b. Most of the solvent from which CO2 has been stripped is returned to the CO2 absorption tower 208 through a pump 202d. The rest of the solvent is introduced into a solvent regeneration tower 211 through a pump 202e and a heat exchanger 212a. The solvent regenerated in the solvent regeneration tower 211 is returned to the CO2 absorption tower 208, and is reused. The CO2 stripping tower 210 and the solvent regeneration tower 211 are generally formed as an integrated tower.
In such a chemical absorption process, when H2S is removed in the acidic gas absorption tower 201, CO2 is also removed and eventually exhausted outside the system. Accordingly, when a low grade coal is used as a raw material, the CO2 partial pressure in the crude gas becomes high, which results in a significant CO2 loss in the H2S removal step. Accordingly, there is a problem that, in the CO2 recovery step in the downstream, recovery ratio of carbon does not reach 90% or more of total carbon. However, when compared to the physical absorption process, the chemical absorption process is advantageous in that the necessary power is low, and in that the purity of the recovered CO2 is high because CO2 is absorbed by a chemical reaction.
U.S. Patent Application Publication No. 2006/0156923 discloses a configuration for separating acidic gas by such a chemical absorption process.